Frequency correction in a multi-carrier communication system

ABSTRACT

A mobile station operable to perform cell synchronization is described. The mobile station can process one or more synchronization signals received in a downlink from one or more base stations providing coverage in one or more cells. The mobile station can process the one or more synchronization signals received from the one or more base stations to synchronize the mobile station with the one or more base stations. The mobile station can adjust signals for communication from the mobile station in accordance with the cell synchronization performed at the mobile station.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.14/992,903, filed Jan. 11, 2016, which is a continuation of U.S. patentapplication Ser. No. 14/017,245, filed Sep. 3, 2013, which is acontinuation of U.S. patent application Ser. No. 13/154,331, filed Jun.6, 2011, which is a continuation of U.S. patent application Ser. No.11/908,253, filed Oct. 30, 2008, which is a National Stage ofInternational Application No. PCT/US06/61881, filed Dec. 11, 2006, whichclaims the benefit of U.S. Provisional Patent Application No.60/749,072, filed on Dec. 9, 2005, each of which are hereby incorporatedby reference in their entirety.

TECHNICAL FIELD

The disclosed technology relates, in general, to wireless communicationsystems and, in particular, to frequency correction in multi-carriercommunication systems.

BACKGROUND

In a multi-carrier communication system, such as an Orthogonal FrequencyDivision Multiple Access (OFDMA) system, the communicated signalconsists of multiple subcarriers (also termed “tones”) that are designedto be mutually orthogonal when sampled at the right frequency points.Such orthogonality can be distorted by a number of factors, one of whichis frequency error. In general, there are two potential sources offrequency error; namely, clock frequency error and Doppler shift. Theclock frequency error is the difference in the clock frequency between amobile device and its serving base station. Normally, the clock at thebase station serves as the reference, to which the clock of a mobiledevice must be synchronized. The Doppler shift is caused by the movementof a mobile device relative to the base station, and the amount of shiftdepends on the speed and direction of the mobile device with respect tothe base station.

The composite frequency error (i.e., the sum of all frequency errorsincluding any clock frequency error and Doppler shift) can be correctedif known. In the downlink (DL) case, the composite frequency error canbe estimated based on the downlink signals and corrected by the receiverat the mobile device. In the uplink (UL) case, since the signalsreceived by the base station consist of signals transmitted by multiplemobile devices, the composite frequency error is a mixture of frequencyerrors from different mobile devices. It may take a very complex processto mitigate the adverse effects of the combined errors at the basestation. It would therefore be beneficial to develop an improved methodof correcting for frequency errors in an environment with multiplemobile devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the coverage of a wireless communication network thatis comprised of a plurality of cells.

FIG. 2 is a block diagram of a receiver and a transmitter, such as mightbe used in a multi-carrier wireless communication network.

FIG. 3 is a graphical depiction of a multi-carrier signal structure inthe frequency domain.

FIG. 4 is a graphical depiction of a multi-carrier signal structure inthe time domain.

FIG. 5 is a graphical depiction a mobile device receiving downlinksignals transmitted by multiple base stations.

FIG. 6 is a graphical depiction of overlaying a special signal componentin the form of a spread-spectrum signal on an OFDM symbol in thefrequency domain.

FIG. 7 is a graphical depiction of a preamble occupying a portion of achannel bandwidth and an entire channel bandwidth.

FIG. 8 is a block diagram of a spatial-temporal processor.

FIG. 9 is a communication diagram of a closed-loop process fordetermining an uplink composite frequency error.

FIG. 10 is a block diagram of a signal conditioning circuit thatutilizes a hybrid approach to frequency error correction.

FIG. 11 is a block diagram of a signal conditioning circuit thatutilizes a digital approach to frequency error correction.

DETAILED DESCRIPTION

Methods and systems for correction of frequency errors in multi-carriercommunication systems are disclosed. Frequency errors for both downlink(DL) and uplink (UL) are corrected at a mobile device based on estimatesof Doppler shift and clock frequency error, using either a hybrid(analog and digital) method or a purely-digital method to compensate forthe frequency error in subsequent communications.

In some embodiments, downlink signals transmitted by a base stationinclude a special signal component in the format of a preamble,midamble, postamble, code sequence, pilots, or controlchannel/subchannel in either the frequency or time domain that isdesigned to facilitate frequency-error estimation and other systemcontrol functionalities.

In some embodiments, a mobile device carries out temporal, spatial, orspatial-temporal processing of the composite frequency errors associatedwith one or more base stations to determine the clock frequency errorand the Doppler shift with respect to its serving base station.

In some embodiments, a clock frequency error calibration table is storedin the mobile device, where the clock frequency error is tabulated as afunction of an operational temperature and other factors.

In some embodiments, a mobile device sends a signal in a particularformat to a base station to allow the base station to estimate thecomposite frequency error. After estimating the composite frequencyerror, the base station transmits information about the compositefrequency error to the mobile device via a particular channel/subchannel(dedicated or otherwise), and the mobile device extracts the informationand utilizes the information to pre-compensate for composite frequencyerrors in subsequent transmissions.

The following discussion contemplates the application of the disclosedtechnology to a multi-carrier system, such as Orthogonal FrequencyDivision Multiplexing (OFDM), Orthogonal Frequency Division MultipleAccess (OFDMA), or Multi-Carrier Code Division Multiple Access(MC-CDMA). The invention can be applied to either Time DivisionDuplexing (TDD) or Frequency Division Duplexing (FDD). Without loss ofgenerality, OFDMA is therefore only used as an example to illustrate thepresent technology.

The following description provides specific details for a thoroughunderstanding of, and enabling description for, various embodiments ofthe technology. One skilled in the art will understand that thetechnology may be practiced without many of these details. In someinstances, well-known structures and functions have not been shown ordescribed in detail to avoid unnecessarily obscuring the description ofthe embodiments of the technology. It is intended that the terminologyused in the description presented below be interpreted in its broadestreasonable manner, even though it is being used in conjunction with adetailed description of certain embodiments of the technology. Althoughcertain terms may be emphasized below, any terminology intended to beinterpreted in any restricted manner will be overtly and specificallydefined as such in this Detailed Description section.

I. Wireless Communication Network

FIG. 1 is a representative diagram of a wireless communication network100 that services a geographic region. The geographic region is dividedinto a plurality of cells 102, and wireless coverage is provided in eachcell by a base station (BS) 104. One or more mobile devices 108 may befixed or may roam within the geographic region covered by the network.The mobile devices are used as an interface between users and thenetwork. Each base station is connected to the backbone of the network,usually by a dedicated link. A base station serves as a focal point totransmit information to and receive information from the mobile deviceswithin the cell that it serves by radio signals. Those skilled in theart will appreciate that if a cell is divided into sectors 106, from asystem engineering point of view each sector can be considered as acell. In this context, the terms “cell” and “sector” areinterchangeable.

In a wireless communication system with base stations and mobiledevices, the transmission from a base station to a mobile device iscalled a downlink (DL) and the transmission from a mobile device to abase station is called an uplink (UL). FIG. 2 is a block diagram of arepresentative transmitter 200 and receiver 220 that may be used in basestations and mobile devices to implement a wireless communication link.The transmitter comprises a channel encoding and modulation component202, which applies data bit randomization, forward error correction(FEC) encoding, interleaving, and modulation to an input data signal.The channel encoding and modulation component is coupled to a subchanneland symbol construction component 204, an inverse fast Fourier transform(IFFT) component 206, a radio transmitter component 208, and an antenna210. Those skilled in the art will appreciate that these componentsconstruct and transmit a communication signal containing the data thatis input to the transmitter 200. Other forms of transmitter may, ofcourse, be used depending on the requirements of the communicationnetwork.

The receiver 220 comprises an antenna 234, a reception component 232, aframe and synchronization component 230, a fast Fourier transformcomponent 228, a frequency, timing, and channel estimation component226, a subchannel demodulation component 224, and a channel decodingcomponent 222. The channel decoding component de-interleaves, decodes,and derandomizes a signal that is received by the receiver. The receiverrecovers data from the signal and outputs the data for use by the mobiledevice or base station. Other forms of receiver may, of course, be useddepending on the requirements of the communication network.

FIG. 3 is a signal diagram depicting the various subcarriers andsubchannels that are contained within a given channel. There are threetypes of subcarriers: (1) data subcarriers, which carry informationdata; (2) pilot subcarriers, whose phases and amplitudes arepredetermined and made known to all receivers, and which are used forassisting system functions such as estimation of system parameters; and(3) silent subcarriers, which have no energy and are used for guardbands and as a DC carrier. The data subcarriers can be arranged intogroups called subchannels to support scalability and multiple-access.The subcarriers forming one subchannel may or may not be adjacent toeach other. Each mobile device may use some or all of the subchannels.

FIG. 4 depicts the basic structure of a multi-carrier signal in the timedomain, which is generally made up of time frames 400, subframes 402,time slots 404, and OFDM symbols 406. A frame consists of a number oftime slots, and each time slot is comprised of one or more OFDM symbols.The OFDM time domain waveform is generated by applying aninverse-fast-Fourier-transform (IFFT) to the OFDM signals in thefrequency domain. A copy of the last portion of the time waveform, knownas the cyclic prefix (CP) 408, is inserted in the beginning of thewaveform itself to form the OFDM symbol. In the case of TDD, guardperiods (GP1 410 and GP2 412), are inserted between an uplink (UL)subframe and a downlink (DL) subframe and between a DL subframe and a ULsubframe to account for the time needed to turn on and off transmittersand receivers, as well as radio propagation delay.

II. Frequency Correction in a Multi-Carrier Communication System AFrequency Errors

There are two potential sources of frequency errors that will adverselyaffect the signal quality of an OFDM system; namely, clock frequencyerror and Doppler shift. The clock frequency error is the difference inthe clock frequency between the mobile device and its serving basestation. Normally, the clock at the base station serves as thereference, to which the clock of a mobile device must be synchronized.The Doppler shift is caused by the movement of a mobile device relativeto the base station, and the amount of shift depends on the speed anddirection of the mobile device with respect to the base station.

In the downlink case where the base station transmits a signal to themobile device, the received signal at the mobile device is characterizedby the following equation (1):

r _(DL)(t)=s _(DL)(t)e ^(j2π(f) ⁰ ^(+f) ^(d) ^()t) e ^(−j2π(f) ⁰ ^(+f)^(e) ^()t) =s _(DL)(t)e ^(j2π(f) ^(d) ^(+f) ^(e) ^()t) =s _(DL)(t)e^(j2πδf) ^(DL) ^(t)

where f₀ denotes the carrier frequency of the transmitted signal fromthe base station, f_(e) denotes the clock frequency error of the mobiledevice with respect to that of the base station, and f_(d) denotes theDoppler shift. The downlink composite frequency error is thereforerepresented by of δf_(DL)=f_(d)−f_(e), which should be corrected at themobile device receiver.

In the uplink case where the mobile device transmits a signals_(UL)(t)e^(j2π(f) ⁰ ^(+f) ^(e) ^()t), the received signal at the basestation is characterized by the following equation (2):

r _(UL)(t)=s _(UL)(t)e ^(j2π(f) ⁰ ^(+f) ^(e) ^(+f) ^(d) ^()t) e ^(−j2πf)⁰ ^(t) =s _(UL)(t)e ^(j2π(f) ^(d) ^(+f) ^(e) ^()t) =s _(UL)(t)e ^(j2πδf)^(UL) ^(t)

Wherein each of the variables is the same as in equation (1) andδf_(UL)=f_(d)+f_(e) denotes the uplink composite frequency error. In anOFDMA system, the signals received by the base station are normally thecombination of signals from multiple mobile devices. Because the uplinkcomposite frequency error (δf_(UL)) in the signal from each mobiledevice is typically different, a complex process would be necessary tomitigate the adverse effects of the uplink composite frequency errors atthe base station. Rather than attempting to mitigate the adverse effectsof the uplink composite frequency errors at the base station, therefore,the disclosed technology pre-compensates for the error at each mobiledevice before transmission.

The clock frequency error depends on a number of external factors, suchas the applied voltage of the power supply and the operating temperatureat the mobile device, and is a time-variant parameter f_(e)(t). Incommon practice, a voltage regulator is used to stabilize the powersupply to the clock. Furthermore, in a normal operating environment, thetemperature does not change rapidly and drastically. Therefore, it isexpected that f_(e)(t) varies relatively slowly and its coherence time(τ_(e)) is in the order of minutes or greater. It follows that theaverage of the clock frequency error over a certain period of time T canbe modeled by the following equation (3):

${{{clock\_ frequency}{\_ error}} = {{\frac{1}{T}{\int_{0}^{T}{{f_{e}(t)}{dt}}}} = \overset{\_}{f_{e}}}},{{where}\mspace{14mu} T{\tau_{e}}}$

The Doppler shift is a time-variant and spatial-variant parameterf_(d)(t) that varies depending on the relative motion of the mobiledevice with respect to the base station. In a typical urban environment,the speed of a mobile device is low and it may change directionsfrequently, and strong multipath signals arrive at the mobile devicewith different values of Doppler shift. In a suburban environment, amobile device tends to move in one direction for longer period of time,but its speed is relatively high which thereby causes a variation inDoppler shift. The fast movement of a mobile device may also causefrequent handover from one base station to the next and the mobiledevice will therefore experience a sudden change in the Doppler shift asthe handoff occurs. Therefore, in a normal operating conditions, theDoppler shift may vary relatively fast and its coherence time (τ_(d)) isin the order of seconds. It follows that the average of the Dopplershift over a certain period of time T can be modeled by the followingequation (4):

${{{{Doppler\_ shift} = {{\frac{1}{T}{\int_{0}^{T}{{f_{d}(t)}{dt}}}} = 0}},{{where}\mspace{14mu} T}}}\tau_{d}$

At any instant in time, a mobile device within a cellular network cannormally detect multiple values of Doppler shift associated with thesignals from multiple base stations. FIG. 5 depicts a mobile device 508that may be in communication with base station 502, base station 504,and base station 506. Because the mobile device's motion relative toeach base station is different, the value of Doppler shift({f_(d,k)}_(k=1) ^(K))) associated with each base station will bedifferent. The downlink composite frequency error associated with thek^(th) base station is expressed by the following equation (5):

δf _(DL,k)(t)=f _(d,k)(t)−f _(e)(t)

Since the speed and direction of the moving mobile device is differentwith respect to each of the base stations, the values of Doppler shiftassociated with each base station is different and tend to cancel eachother out if summed.

B. Frequency Error Estimation

To enable a mobile device to pre-compensate for frequency errors insubsequent transmissions, the device utilizes an estimate of thecomposite frequency error (δf_(UL)) or both the Doppler shift (f_(d)(t))and clock frequency error (f_(e)). Such estimates may be determinedusing a variety of different techniques. To facilitate frequency-errorestimation and other system control functionalities, a base station maytransmit a special signal component in the format of a preamble,midamble, postamble, code sequence, pilots, or controlchannel/subchannel in either the frequency or time domain. The specialsignal component transmitted by a base station is orthogonal ornear-orthogonal in frequency, time, or code to those transmitted byother base stations that are located in and provide service to adjacentcells. Furthermore, the signal component is transmitted by the basestations periodically, for example, within an OFDM symbol, in every slotor every multiple slots, and/or in every frame or every multiple frames.

In some embodiments, the special signal component is overlaid on othertypes of signals in the time or frequency domain (e.g., on an OFDMsymbol in the frequency domain as shown in FIG. 6) and may occupy theentire or partial bandwidth of the channel, as depicted by the examplesshown in FIG. 7. When the special signal component only partiallyoccupies an available bandwidth, the occupied portion does not have tobe contiguous.

In some embodiments, a group of base stations may transmit the identicalspecial signal component simultaneously to facilitate the frequencyerror estimation by the mobile devices. For example, common pilotsubcarriers may be transmitted by a group of base stations within thesame OFDM symbol period.

In determining the clock frequency error and the Doppler shift withrespect to its serving base station, a mobile device estimates, based onthe special signal components it receives from a group of base stations,the composite frequency errors with respect to these base stations. Themobile device then carries out temporal, spatial, or spatial-temporalprocessing of the estimated errors associated with the group of basestations to determine the clock frequency error and the Doppler shiftwith respect to its serving base station.

FIG. 8 is a block diagram depicting inputs to a processor 802 that maybe used to implement spatial-temporal, temporal-only, or special-onlyprocessing. In some embodiments the estimates of the composite frequencyerrors associated with K base stations over N temporal samples are inputto the processor 802 for spatial-temporal processing, where N is chosensuch that δ_(d)<<T<<δ_(e). In practice, N can be in the order ofhundreds of slots or tens of frames. Signal attributes with respect totime and individual base stations, such as signal strength and/or SNR,are also input to the processor 802 and utilized in the processing. Theoutput of the processor 802 is an estimate of the clock frequency error.The processor 802 may consist of memory and coding circuits to realize amulti-dimensional low-pass filter (LPF) of a particular form, such as afinite impulse response (FIR), infinite impulse response (IIR),adaptive, linear, or non-linear filter with its pass-bandwidth less thanthe bandwidth of f_(d)(t) and greater than that of f_(e)(t). The signalattributes are used in the spatial-temporal process to obtain theresults in the optimal statistical sense, such as maximum SNR, maximumlikelihood, or minimum mean-squared error.

When the processor 802 is configured to implement a linear FIR filter,the output of the processor can be expressed by the following equation(6):

${\overset{\sim}{f}}_{e} = {- {\sum\limits_{k = 1}^{K}{\sum\limits_{n = 1}^{N}{\alpha_{k,n}\beta_{k,n}\delta \; {f_{{DL},k}(n)}}}}}$

where α_(k,n) is the filter coefficient and β_(k,n) is the signalattribute with respect to the time index (n) and the base station index(k). The estimate of the Doppler shift with respect to its serving basestation (for example, Base Station 1 in FIG. 5) is then expressed by thefollowing equation (7):

f _(d,1)(n)=δf _(DL,1)(n)+{tilde over (f)} _(e)

Alternatively, the uplink composite frequency error is expressed by thefollowing equation (8):

δf _(UL,1)(t)=δf _(DL,1)(t)+2{tilde over (f)} _(e)

The spatial-temporal processing can further be realized by using atwo-dimensional averaging or low-pass filter with its coefficientsweighted by the corresponding signal attributes, such as signal strengthand/or SNR. In some embodiments, the spatial-temporal processing isrealized by using first a one-dimensional averaging or low-pass filterin the time domain and then a second one-dimensional averaging orlow-pass filter in the spatial domain with the coefficients of thefilters weighted by the appropriated signal attributes.

In some embodiments, the estimates of the composite frequency errors areinput to a processor 802 for temporal-only processing to determine theclock frequency error. The estimates of the composite frequency errorsassociated with K base stations over N temporal samples are input to theprocessor for temporal averaging or filtering, where N is chosen suchthat δ_(d)<<T<<δ_(e). Signal attributes with respect to time can be usedto weight the coefficients of the filter. Temporal-only processing canbe used in applications or situations such as:

-   -   1. when a group of base stations transmit the same signal (e.g.        in a single frequency network);    -   2. when a group of base stations transmit the same special        signal component;    -   3. when signals from only one base station are available;    -   4. when the signals from one base station are dominant in signal        strength; or    -   5. when the mobile device is configured to work with only the        signals from its serving base station.

In some embodiments, the estimates of the composite frequency errors areinput to a processor 802 for spatial-only processing to determine theclock frequency error. In spatial-only processing, the estimates of thecomposite frequency errors associated with K base stations at an instantare input to the processor 802 for spatial averaging or filtering, withthe filter coefficients weighted by the signal attributes associatedwith the K respective base stations.

In some embodiments, the spatial or spatial-temporal processing alsoapplies to cases where the mobile device is capable of simultaneouslyreceiving multiple signals through multiple receivers.

In some embodiments, instead of inferring the Doppler shift from thecomposite frequency error the Doppler shift can be directly computedbased on the information of the movement (including speed and direction)of the mobile device relative to its serving base station. Movement ofthe mobile device can be derived from a location component (e.g., globalpositioning system (GPS) device) that is integrated in the mobile deviceor in another system (e.g., an automobile) that is in communication withthe mobile device, from triangulation from one or more signals receivedfrom base stations, or by using any other method that allows themovement of the mobile device to be tracked.

In some embodiments, the clock of a mobile device can be calibrated bymaintaining a clock frequency error calibration table. The clockfrequency error calibration table is stored in the mobile device, andused to maintain a record of the clock frequency error as a function ofthe operational temperature and other factors. The values of such acalibration table are initialized during manufacturing testing or arepreset with default values. The calibration table can be updated basedon the estimate of the clock frequency error ({tilde over (f)}_(e)) andthe corresponding operational temperature setting that is supplied by athermo-sensor. Given a current operational temperature, thecorresponding clock frequency error can be identified in the calibrationtable and used directly or in conjunction with the current estimates ofthe clock frequency error to correct the frequency error.

In some embodiments, the composite frequency error may be determined forthe uplink using a closed-loop process. In the closed-loop process themobile device sends a signal in a particular format to the base stationto allow the base station to estimate, by a certain method, thecomposite frequency error. The base station transmits informationpertaining to the composite frequency error to the mobile device via aparticular channel/subchannel (dedicated or otherwise); and the mobiledevice extracts the information and uses the information topre-compensate for frequency error during the subsequent transmission ofsignals.

In the closed-loop process, the signal that the mobile device sends tothe base station to enable the estimation of the composite frequencyerror is coded either in the frequency domain or in the time domain andoccupies all of or a portion of the channel. The signal can be speciallydesigned for frequency estimation or can be a general purpose signalsuch as a ranging signal.

The information of the composite frequency error transmitted by the basestation to the mobile device can be in the form of an actual frequencyerror value, an incremental value, an explicit value, an implicit value,or any other suitable format and can be represented in a dedicated bitfield or embedded in a data field. The transmitted information can beencoded or uncoded.

A tracking filter may be applied to the estimates of the compositefrequency error to smooth out noise components. The mobile device mayperiodically send the enabling signal to the base station so as toupdate the frequency error information.

An example of a specific closed-loop process is illustrated in thecommunication diagram of FIG. 9. The communication flow depicted in FIG.9 implements the following functions:

-   1. When powered on, the mobile device searches for a serving base    station and then carries out frequency synchronization based on the    downlink signals (e.g., preambles) from the base station by    estimating and tracking the downlink composite frequency error    (δf_(DL)).-   2. The mobile device sends a signal, such as a ranging signal, to    the base station to facilitate the estimation of the frequency    error. The mobile device may or may not pre-correct the signal phase    by (δ_(DL)).-   3. Based on the signal sent by the mobile device, the base station    carries out, using a particular signal processing technique, the    estimation of the frequency error. In the case without any    pre-correction, the error is δf_(UL) and in the case of    pre-correction with (δf_(DL)), the error becomes δf_(UL)−δf_(DL).    Once the error is determined, the base station transmits the error    information to the mobile device.-   4. The mobile device extracts the error information, from which the    composite frequency error (δf_(UL)) is inferred and applied to    pre-compensate the signal for the frequency error before    transmission.

C Frequency Correction

Regardless of which techniques are used to estimate the Doppler shiftand clock frequency error for the mobile device, once the Doppler shiftand clock frequency error have been determined they may be used tocorrect frequency errors for both downlink communications to and uplinkcommunications from the device. The frequency of a received signal maybe corrected for the downlink composite frequency error by rotating thesignal with a phase value equal to the error but in the oppositedirection; as represented by the following equation (7):

r _(DL)(t)e ^(−j2πδf) ^(DL) ^(t) =s _(DL)(t)(e ^(j2πδf) ^(DL) ^(t))(e^(−j2πδf) ^(DL) ^(t))=s _(DL)(t)

The frequency of a signal to be transmitted may be pre-compensated forthe anticipated composite frequency error accrued in the uplinktransmission. The pre-compensation is achieved by rotating the signalwith the phase value equal to the error but in the opposite direction;as represented by the following equation (8):

s′ _(UL)(t)=s _(UL)(t)e ^(−j2πδf) ^(UL) ^(t)

At the base station, the received signal can be recovered without theadverse effect caused by the composite frequency error, as representedby the following equation (9):

r _(UL)(t)==s′ _(UL)(t)e ^(j2π(f) ⁰ ^(+f) ^(e) ^(+f) ^(d) ^()t) e^(−j2πf) ⁰ ^(t) =s _(UL)(t)

In some embodiments, the clock frequency error is corrected by adjustinga local oscillator of the mobile device, whereas the Doppler shift iscompensated for by digitally rotating the phase of the uplink ordownlink signals appropriately in the time domain. A specific example isgiven in FIG. 10 of this hybrid approach to composite frequency errorcorrection. FIG. 10 is a block diagram of a signal conditioning circuitthat operates in the analog and digital domains in order to correct forcomposite frequency errors. The clock frequency error is corrected inthe analog domain by adjusting a local oscillator 1002 using a control1004 to modify the oscillation of the local oscillator by an amountequal to the estimated clock frequency error f_(e). The Doppler shift iscorrected in the digital domain by rotating the phase of a signal to betransmitted using a digital multiplier 1006 (to pre-compensate signalsfor transmission) or by rotating the phase of a signal that has beenreceived using a digital multiplier 1008 (to compensate receivedsignals).

In some embodiments, the clock frequency error and the Doppler shift arecompensated for by digitally rotating the phase of the signalsappropriately in the time domain. A specific example is given in FIG. 11of this purely digital approach to composite frequency error correction.FIG. 11 is a block diagram of a signal conditioning circuit thatoperates in the digital domain in order to correct for compositefrequency errors. The clock frequency error and the Doppler shift arecorrected in the digital domain by rotating the phase of a signal to betransmitted using a digital multiplier 1106 (to pre-compensate signalsfor transmission), or by rotating the phase of a signal that has beenreceived using a digital multiplier 1108 (to compensate receivedsignals). When signal processing is performed in the digital domain, thelocal oscillator 1102 is not adjusted since the estimated clockfrequency error is corrected by the multipliers.

While both the Doppler shift and clock frequency error may be estimatedto correct for composite frequency errors, in some environments it maybe beneficial to estimate and correct for only the Doppler shift or onlythe clock frequency error. Representative environments where this may bebeneficial include, but are not limited to, those environments where themobile device is stationary or the clock frequency error is minimal.

The above detailed description of embodiments of the system is notintended to be exhaustive or to limit the system to the precise formdisclosed above. While specific embodiments of, and examples for, thesystem are described above for illustrative purposes, various equivalentmodifications are possible within the scope of the system, as thoseskilled in the relevant art will recognize. For example, while processesare presented in a given order, alternative embodiments may performroutines having steps in a different order, and some processes may bedeleted, moved, added, subdivided, combined, and/or modified to providealternative or subcombinations. Each of these processes may beimplemented in a variety of different ways. Further any specific numbersnoted herein are only examples: alternative implementations may employdiffering values or ranges.

These and other changes can be made to the invention in light of theabove Detailed Description. While the above description describescertain embodiments of the technology, and describes the best modecontemplated, no matter how detailed the above appears in text, theinvention can be practiced in many ways. Details of the system may varyconsiderably in its implementation details, while still beingencompassed by the technology disclosed herein. As noted above,particular terminology used when describing certain features or aspectsof the technology should not be taken to imply that the terminology isbeing redefined herein to be restricted to any specific characteristics,features, or aspects of the technology with which that terminology isassociated. In general, the terms used in the following claims shouldnot be construed to limit the invention to the specific embodimentsdisclosed in the specification, unless the above Detailed Descriptionsection explicitly defines such terms. Accordingly, the actual scope ofthe invention encompasses not only the disclosed embodiments, but alsoall equivalent ways of practicing or implementing the invention underthe claims.

What is claimed is:
 1. An apparatus of a mobile station operable toperform cell synchronization, the apparatus comprising: memory; and oneor more processors configured to: process, at the mobile station, one ormore synchronization signals received in a downlink from one or morebase stations providing coverage in one or more cells; process, at themobile station, the one or more synchronization signals received fromthe one or more base stations to synchronize the mobile station with theone or more base stations; and adjust, at the mobile station, signalsfor communication from the mobile station in accordance with the cellsynchronization performed at the mobile station.
 2. The apparatus ofclaim 1, further comprising a receiver configured to receive the one ormore synchronization signals in the downlink from the one or more basestations.
 3. The apparatus of claim 1, wherein the one or moresynchronization signals include a first synchronization signal and asecond synchronization signal.
 4. The apparatus of claim 1, wherein theone or more processors are further configured to process the one or moresynchronization signals to perform cell synchronization in a frequencydomain.
 5. The apparatus of claim 1, wherein the one or more processorsare further configured to process the one or more synchronizationsignals to perform cell synchronization in a time domain.
 6. Theapparatus of claim 1, wherein the one or more synchronization signalsare code sequences.
 7. The apparatus of claim 1, wherein the one or moresynchronization signals are received at the mobile station periodicallyin multiple slots in an orthogonal frequency division multiplexing(OFDM) symbol.
 8. At least one non-transitory machine readable storagemedium having instructions embodied thereon for performing cellsynchronization at a mobile station, the instructions when executed byone or more processors at the mobile station perform the following:processing, at the mobile station, a first synchronization signalreceived in a downlink from a first base station; processing, at themobile station, a second synchronization signal received in a downlinkfrom a second base station; processing, at the mobile station, the firstsynchronization signal and the second synchronization signal more basestations to synchronize with the first and second base stations; andadjusting, at the mobile station, signals for communication from themobile station in accordance with the cell synchronization performed atthe mobile station.
 9. The at least one non-transitory machine readablestorage medium of claim 8, wherein the first synchronization signal andthe second synchronization signal that occupy a center portion of achannel bandwidth.
 10. The at least one non-transitory machine readablestorage medium of claim 8, wherein the cell synchronization is performedat the mobile station in a frequency domain.
 11. The at least onenon-transitory machine readable storage medium of claim 8, wherein thecell synchronization is performed at the mobile station in a timedomain.
 12. The at least one non-transitory machine readable storagemedium of claim 8, wherein the first synchronization signal and thesecond synchronization signals are code sequences.
 13. The at least onenon-transitory machine readable storage medium of claim 8, wherein thefirst synchronization signal is received at the mobile stationperiodically in multiple slots in an orthogonal frequency divisionmultiplexing (OFDM) symbol.
 14. At least one non-transitory machinereadable storage medium having instructions embodied thereon forfacilitating cell synchronization for a mobile station at a basestation, the instructions when executed by one or more processors at thebase station perform the following: processing, from the base station,one or more synchronization signals for transmission to the mobilestation in a downlink, wherein the one or more synchronization signalscause the mobile station to synchronize with the base station; andprocessing, at the base station, communication signals received from themobile station, wherein the communication signals are adjusted at themobile station for communication in accordance with the cellsynchronization performed at the mobile station.
 15. The at least onenon-transitory machine readable storage medium of claim 14, wherein theone or more synchronization signals are transmitted periodically to themobile station in multiple slots in an orthogonal frequency divisionmultiplexing (OFDM) symbol.
 16. The at least one non-transitory machinereadable storage medium of claim 14, wherein the one or moresynchronization signals transmitted from the base station include afirst synchronization signal and a second synchronization signal. 17.The at least one non-transitory machine readable storage medium of claim14, wherein the one or more synchronization signals transmitted from thebase station include a first synchronization signal and a secondsynchronization signal, wherein the first synchronization signal isnear-orthogonal to the second synchronization signal.
 18. The at leastone non-transitory machine readable storage medium of claim 14, whereinthe one or more synchronization signals transmitted from the basestation include a first synchronization signal and a secondsynchronization signal that occupy a partial bandwidth of a channel oran entire bandwidth of the channel.
 19. The at least one non-transitorymachine readable storage medium of claim 14, wherein the one or moresynchronization signals are transmitted to the mobile station to causethe mobile station to perform cell synchronization in a frequencydomain.
 20. The at least one non-transitory machine readable storagemedium of claim 14, wherein the one or more synchronization signals aretransmitted to the mobile station to cause the mobile station to performcell synchronization in a time domain.
 21. The at least onenon-transitory machine readable storage medium of claim 14, wherein theone or more synchronization signals transmitted from the base stationare code sequences.